Rechargeable battery have come into widespread use not only for digital household electrical appliances but also for motor-powered electric vehicles and hybrid vehicles.
A redox-flow battery, which performs charging/discharging based on the change in oxidation numbers, using two redox pairs that produce reduction-oxidation (redox) reaction in an electrolyte, with vanadium used as an active material, is known (Patent Reference 1).
In particular, a vanadium redox-flow battery, which includes +2 and +3 vanadium ions in oxidation states, namely V2+ and V3+, and +4 and +5 vanadium ions in oxidation state, namely V4+ and V5+, as redox pairs and supplies vanadium sulfuric acid solution stored in a tank to a flow type cell to cause charging and discharging, is used in the field of large power storage.
A redox-flow battery includes an positive electrolyte tank and a negative electrolyte tank respectively containing positive electrolyte, namely an active material on the positive electrode side, and negative electrolyte, namely an active material on the negative electrode side, and a stack for performing charging/discharging. The positive electrolyte and the negative electrolyte are transferred from the positive electrolyte tank and the negative electrolyte tank to the stack, and circulated. The stack has a structure where an ion-exchange membrane is sandwiched between the positive electrode and the negative electrode, and the following formulae represent the reactions that occur in the positive electrolyte and the negative electrolyte of the battery.Positive electrode: VO2+(aq)+H2OVO2+(aq)+e−+2H+  [Chemical formula 1]Negative electrode: V3+(aq)+e−V2+(aq)  [Chemical formula 2]
In the above [Chemical formula 1] and [Chemical formula 2], “” represents chemical equilibrium, and the suffix (aq) of ions indicates that the ions exist in the solution.
Since all the above reactions occur in sulfuric acid solutions, namely positive electrolyte and negative electrolyte, performance degradation due to generation of dendrite on the surface of electrodes does not occur at the time of charging, unlike lead battery using metal electrodes, and charging/discharging reactions can be repeated infinitely in principle.
In −zero charged state, namely the state where the positive electrolyte contains V4+(aq) only and the negative electrolyte contains V3+(aq) only, the open-circuit voltage of the battery is approximately 1.1 V.
When a sufficiently large voltage is applied between the positive electrode and the negative electrode using an external power supply to forcibly feed current to a vanadium redox-flow battery for charging, V4+(aq) in the positive electrolyte is oxidized to V5+(aq), and at the same time V3+(aq) in the negative electrolyte is reduced to V2+(aq). If charging is completed and 100% charged state is reached, the open-circuit voltage of the battery becomes approximately 1.58 V.
The capacity of vanadium redox-flow battery is determined based on the amount of vanadium dissolved in electrolyte. For example, in the case of a vanadium redox-flow battery containing two different electrolytes having given molar concentrations, the battery capacity is directly proportional to the volume of these two electrolytes. In other words, if the concentration of the positive electrolyte and the negative electrolyte is increased, and/or the volume of the positive electrolyte and the negative electrolyte is increased, the battery capacity increases. The volume of the positive electrolyte and the negative electrolyte can be increased by increasing the volume of the positive electrode tank and the negative electrode tank.
The energy density is an another specification, other than battery capacity, characterizing the performance of battery. Energy density is defined as the amount of energy (electric energy) that can be produced per unit weight of battery. For example, a lithium-ion rechargeable battery is a typical rechargeable battery that produces electric power using oxidation/reduction reactions that occur at electrodes. One of the reasons why lithium is used is that lithium is a light metal (atomic weight: 6.94), which is advantageous in achieving high energy density.
Since redox-flow battery uses solutions as electrolytes, their energy density is generally low. To improve this, a cerium-chromium redox gel battery is proposed (Patent Reference 2). This redox gel battery includes an inert anode; an inert cathode; a positive redox-gel electrolyte containing cerium chloride for example and contacting the inert anode; a negative redox-gel electrolyte containing chromium chloride for example and contacting the inert cathode; and a separator placed between the positive and negative redox-gel surfaces on the side opposite to the surfaces respectively contacting the anode and the cathode.
With this cerium-chromium redox-gel battery, the positive and negative gels are respectively made of Ce4+ and Cr2+ in charged state. In discharged state, the negative gel electrolyte Cr2+ is oxidized to Cr3+, whereas the positive gel electrolyte Ce4+ is reduced to Ce3+.
In addition, to obtain a compact and light-weight redox battery having high output performance compared to a redox-flow battery, in particular, an stationary-electrolyte-type redox battery is proposed (Patent Reference 3). The positive electrode electrolyte solution for the positive electrode and electrolyte solution tank for negative electrode of this stationary-electrolyte-type redox battery is filled with an electrolyte mixed with electrodes, namely a mixture of electrolyte and powder or small pieces of a conductive material.
More specifically, the stationary-electrolyte-type redox battery without an electrolyte storage tank is known including at least a separator; electrolyte tanks on the positive and negative electrode sides; bipolar plates on the positive and negative electrode sides; a metal plate having a positive electrode terminal, and a metal plate having a negative electrode terminal, wherein the electrolyte tanks on the positive and negative electrode sides are filled with a mixture of an electrolyte containing vanadium ions as an active material and a conductive material operating as electrodes, powder or small pieces of carbon for example.